Method of making an optical information recording medium and method of recording/reproducing optical information

ABSTRACT

A rewritable optical information recording medium of phase-change type wherein the recording, erasing, reproduction and rewriting of information are effected by irradiation of a high density energy flux such as laser beams. With the aim of obtaining a high erasing ratio in overwriting using a single laser beam, a constitution of medium has been devised whereby the same temperature-rise profile can be obtained for both the recorded mark part and the unrecorded (erased) part of the recording film. For example, by selecting the film thickness of each layer such that the optical absorbance at the wavelength of irradiation light source is the same in both the recorded part and the unrecorded part, an erasing ratio of -30 dB or more has been attained.

This is a division of application Ser. No. 07/997,640, filed Dec. 28,1992, now U.S. Pat. No. 5,273,861, which is a continuation of Ser. No.07/663,798, filed Mar. 4, 1991 (abandoned), which is a continuation ofSer. No. 07/276,630, filed Nov. 28, 1988 (abandoned).

BACKGROUND OF THE INVENTION

The present invention relates to a rewritable optical informationrecording medium of phase-change type wherein the recording, erasing,reproduction, and rewriting of information are effected by irradiationof a high-density energy flux such as laser beams.

The technique is already known which comprises forming a thin,light-absorbing recording film on a substrate of glass, resin or similarmaterials having a smooth surface, and then irradiating thereonto alaser beam converged into a micro spot to cause a local change ofoptical properties at the irradiated part, and thereby recordingintended information. In such a technique, by using, as the recordingfilm, for example, a thin film of certain kinds of chalcogenide glassbased on Te, Se and the like or a thin film of metals such as AgZn andAuSb, it is possible to make the above-mentioned change of opticalproperties reversible and thereby to perform the recording, erasing andrewriting of information repeatedly. The recording and erasing areeffected based on the difference in optical properties due to thereversible change of structure on an atomic level between the crystalphase and the amorphous phase, or between the high temperature phase andthe low temperature phase of the crystal phase, of respective recordingfilm. Thus, according to one type of operation, record/erase operationsare reversible based on changes between crystal and amorphous phases. Inanother type of operation, record/erase operations are reversible basedon changes between high and low temperature phases of the crystal phase.In either type, the difference in the quantity of reflected light, orthe quantity of transmitted light, of a specific wavelength is detectedas a signal. In other words, the light absorbed in the recording mediumis converted into heat to increase the temperature of the irradiatedpart. In recording, the irradiated part is brought to an elevatedtemperature until it fuses and then is quenched from the fused state,whereby a recorded state in the term of and amorphous state or a hightemperature phase of the crystal phase is obtained. In erasing, thesemetastable phases are heated and maintained in the vicinity of the glasstransition temperature, whereby an erased state in the form of a crystalstate or low temperature phase of the crystal phase is obtained. Betweenthe recorded state and the erased state, there exist differences inoptical constants (e.g. refractive index and extinction coefficient),which can be detected as differences in such optical properties asreflectance and transmittance. In general practice, the recording filmlayer is used in a sandwiched structure with layers of dielectrics suchas SiO₂ and ZnS to avoid vaporization and so forth of the recording filmlayer in repeated use. In the prior art, the thickness of each layer wasselected so as to give an enhanced recording sensitivity, for example,to increase the absolute efficiency of light absorption of the recordinglayer in respective states and to give, at the same time, as wide adifference as possible in the quantity of reflected light or transmittedlight before and after the change. In one example, a light-reflectinglayer of Au, Al and the like was additionally applied onto thedielectric layer of the side opposite to the incident light.

The recording and erasing by means of irradiation of a laser beam ontothe recording medium may be conducted in practice according to either ofthe following two methods. In one method, separate laser beams are usedrespectively for recording and for erasing, and previously recordedsignals are erased by the preceding beam and new signals are recorded bythe succeeding beam (namely, so-called overwriting is conducted). In theother method, a single laser beam is used, whose irradiation power canbe changed on two steps of recording level and erasing level and ismodulated therebetween in response to information signals, and newsignals are directly written on the information track having signalsrecorded thereon (namely,so-called direct overwriting is conducted). Inthe former method, the laser power and irradiation time can be selectedindependently for recording and for erasing and hence no particularproblem due to overwriting occurs. On the other hand, the latter method,which has come to be predominantly used, has the advantage offacilitating the design of optical heads but, on the other hand, bringsabout the following disadvantage. That is, since no previous erasingoperation is conducted before recording, recording marks havingdifferent sizes and atomic ordering are produced in the case ofrecording onto amorphous parts (that is, making the parts amorphousagain) as comprised with the case of recording onto crystal parts. Inother words, a problem occurs wherein the dimensions of recording markschange to some extent in accordance with the state before recording as aresult the signal component which should have been erased before leavessome effect on new signals. The above problem is conceivably caused bythe following two factors. One is the difference in optical absorbanceof the recording layer existing between the amorphous state part and thecrystal state part. The other is the difference in the energy requiredfor melting (latent heat of melting) of the recording layer existingbetween the amorphous state part and the crystal state part.

SUMMARY OF THE INVENTION

An object of the present invention is to provide, as a means for solvingthe above problem, an optical information recording medium wherein therespective film thicknesses of the recording layer, dielectric layer andreflecting layer are so designed as to make the optical absorbance ofthe recording layer in the recorded state and that in the erased stateequal to each other.

Another object of the present invention is to provide an opticalinformation recording medium wherein, in order that the difference inlatent heat of melting (or like properties) between the recording filmin the recorded state and that in the erased state might be cancelledout, the state whose latent heat of melting is higher is made to have ahigher light absorption efficiency than that of the state whose latentheat of melting is lower. Thus, by making the optical absorbance of therecording layer in the two states equal to each other or by making themdiffer from each other enough to counterbalance the difference in latentheat of melting, approximately similar temperature-rise profiles can beobtained for both states and hence the shapes and dimensions ofrecording marks can be made substantially equal. Thus, overwriting ispossible at a high erasing ratio.

It should be noted that as used herein the term "optical absorbance"means the ratio of the absorbed light quantity to the irradiated lightquantity of a material.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 and 2 each show a sectional view of an embodiment of the opticalinformation recording medium according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The optical information recording medium of the present invention is, asshown in FIGS. 1 and 2, constructed by forming a recording layer 2,sandwiched between dielectrics 3 such as SiO₂ and ZnS, on a substrate 1having a smooth surface made of resins such as polymethyl methacrylate(PMMA) and polycarbonate, metals such as Al and Cu, or glass. Thematerial used for constituting the recording layer may be those in whichthe reversible phase change between amorphous state and crystal state ismade use of, typically chalcogenides based on Te and Se, for example,GeTe, InSe, InSeTl, InSeTlCo, GeTeSb, GeTeSn, GeTeSnAu, InTe, InSeTe,InSbTe, SbSeTe and the like, and those in which the reversible phasechange between high-temperature crystalline phase and low-temperaturecrystalline phase, for example, InSb, AuZn, AuSb and the like isutilized. A construction is also possible wherein a light reflectinglayer 4 is additionally provided on the dielectrics layer of the sideopposite to the incident laser beam. For the reflecting layer there maybe used Au, Cu, Al, Ni, Cr, Pt, Pd, and alloys thereof. It is alsopossible to laminate a protecting sheet or plate 5 onto the upper-mostpart by a vacuum deposition method or through an adhesive resin layer.

The essential point of the present invention is to make thetemperature-rise profiles of the recording layer in the two states(recorded state and erased state) before and after recordingsubstantially equal to each other, which can be achieved byappropriately selecting the film thickness of respective layers in theconstruction stated above. The film thickness of respective layers canbe determined, based on the optical constant (refractive index orextinction coefficient), by a calculation using, for example, the matrixmethod described on page 69 of "OPTICAL PROPERTIES OF THIN SOLID FILMS"(written by O.S. Heavens, published by Dover Publication Inc. in 1965).The selection of items to be calculated is a step forward from that inthe prior method; namely, not only the absolute values of opticalabsorbance of the recorded part and the unrecorded part but also therelative relationship between the two values is taken intoconsideration. In other words, conditions are preferentially adoptedwherein the difference between the two values is small even if theirabsolute values are somewhat low or wherein, as will be described later,the absorbance of the more difficultly fusible state is higher.

Thus, it is important that (1) when no difference in internal energyexists between the two states of before and after recording (namely,recorded state and erased state) the optical absorbances of therecording layer in the two states should be made equal to each other and(2) when a difference in the easiness of fusion exists due to adifference of internal energy, the optical absorbance of the moredifficultly fusible state should be made to be relatively higher,thereby to obtain in either state a similar temperature-rise profile inrespect of both time and space. As compared with the amorphous state,the crystal state is low in internal energy (so that its latent heat ofmelting is lower) and hence requires correspondingly higher energy inmelting. Similarly, when the high temperature phase in the crystal phaseis compared with the low temperature phase, the latter phase has a lowerlatent heat of melting and requires a higher energy for melting. Thus,where E represents a quantity of energy required for melting; arepresents an amorphous state and a higher temperature phase; brepresents a crystal state and a lower temperature phase; and E.sub. aand E_(b) represent a quantity of energy required for melting each ofthese substances in state a or b respectively, the relationship orcorrelation between E_(a) and E_(b) is as follows: E_(a) <E_(b).Accordingly, when use is made of the phase change between the amorphousstate and the crystal state or of the phase change between the hightemperature phase in the crystal phase and the low temperature phase,the optical absorbance of the recording layer in the crystal phase or inthe low temperature phase is respectively made relatively higher thanthat in the amorphous phase or high temperature phase, so that therecording layer in the respective former phases may absorb a greateramount of energy. Tables 1(a) and (b) show an embodiment of the presentinvention wherein the recording medium is shaped in the form of aso-called optical disk whose recording layer is formed of GeSb₂ Te₄, thedielectrics layer of ZnS and the reflecting layer of Au. The substrateis polycarbonate and has spiral tracks formed thereon for the lightguide. In this recording film, the latent heat of melting is about 6cal/g higher for the crystalline phase than that of the amorphous phaseand hence it is expected that said difference must be cancelled out bycontrolling the balance of optical absorbance of the recording layer forboth states. It is shown in Tables 1(a) and 1(b) that the respectiveoptical absorbance of the recorded part and the erased part becomeshigher or lower relative to each other depending on the selection of thefilm thickness of respective layers. Evaluations were performed on adynamic tester having a single laser diode of 830 nm in wave length forseveral combinations of these film thicknesses to examine comparativelythe CN ratio and erasing ratio. Table 1 (a) shows some examples of filmthickness constitution and Table 1(b) shows the optical absorbance inthe recording layer and reflectance of the disk before and afterrecording for 830 nm in wave length, as well as the CN ratio and erasingratio for these examples.

Each constitution has the following characteristic. In the Tables,sample Nos. 1, 2 and 3 each have a recording layer of 40 nm thicknessand sample Nos. 4, 5 and 6 a recording layer of 20 nm thickness. In eachsample group, the relationship between the optical absorbance Aa of therecording layer at the amorphous part and the absorbance Ab at thecrystal part was selected so as to be, in the order of the samplenumber, Aa<Ab, Aa=Ab and Aa>Ab. In the determination, recording signalswere overwritten at a linear velocity of 15 m/sec and alternately at afrequency of 7 MHz or 5 MHz. The laser power level was 12-20 mW forrecording (amorphizing) and 5-10 mW for erasing (crystallizing). TheTables show the best values of CN ratio (CNR) and erasing ratio in theabove-mentioned range of power levels at 7 MHz. The Tables reveal thatwhen the optical absorbance of the recording layer in the amorphousstate is higher than that in the crystal state no satisfactory erasingratio is obtained though the CNR is high, and when the opticalabsorbance of the recording layer in the crystal state is equal to orhigher than that in the amorphous state a high CNR and a high erasingratio can be obtained simultaneously.

Thus, according to the optical information recording medium of thepresent invention, it has become possible to conduct overwriting using asingle laser beam while maintaining a high CNR and a high erasing ratio.

                  TABLE 1 (a)                                                     ______________________________________                                        Disk Constitution Examples                                                             Under               Upper                                                     coating Recording   coating                                                                             Reflecting                                 Sample   layer   layer       layer layer                                      No.      ZnS     GeSb.sub.2 Te.sub.4                                                                       ZnS   Au                                         ______________________________________                                        1        86 nm   40 nm       151 nm                                                                              20 nm                                      2        86 nm   40 nm       145 nm                                                                              20 nm                                      3        43 nm   40 nm       140 nm                                                                              20 nm                                      4        86 nm   20 nm       173 nm                                                                              20 nm                                      5        48 nm   20 nm       162 nm                                                                              20 nm                                      6        65 nm   20 nm       162 nm                                                                              20 nm                                      ______________________________________                                    

                  TABLE 1 (b)                                                     ______________________________________                                        Comparison of Characteristic of Each Disk                                                                             Erasing                               Sample           Amor-           CNR    ratio                                 No.              phous    Crystal                                                                              (dB)   (dB)                                  ______________________________________                                        1     Reflectance                                                                              2.9%     22.0%  56 dB  -20 dB                                      Absorbance 70.0%    62.6%                                               2     Reflectance                                                                              6.0%     18.0%  54 dB  -29 dB                                      Absorbance 63.0%    63.0%                                               3     Reflectance                                                                              12.4%    22.1%  52 dB  -34 dB                                      Absorbance 52.9%    57.9%                                               4     Reflectance                                                                              0.4%     16.8%  56 dB  -20 dB                                      Absorbance 73.5%    69.5%                                               5     Reflectance                                                                              3.5%     19.0%  55 dB  -28 dB                                      Absorbence 59.5%    59.5%                                                     Reflectance                                                                              2.9%     14.1%  54 dB  -32 dB                                      Absorbance 60.1%    64.2%                                               ______________________________________                                    

We claim:
 1. A method of making an optional information recording mediumcomprising:(a) laminating successively, on a substrate, a firstdielectric layer, a phase change type recording layer capable ofchanging reversibly between an amorphous state and a crystalline stateformed reversibly in response to laser beam irradiation conditions, asecond dielectric layer, a light reflecting layer and a protectionlayer; and (b) selecting respective thicknesses of said first dielectriclayer, said recording layer, said second dielectric layer, said lightreflecting layer and said protection layer, in accordance withcalculations, to provide a first calculated ratio of light absorptionquantity in the recording layer to light irradiation quantity on therecording medium said recording layer is in said crystalline state and asecond calculated ratio of light absorption quantity in the recordinglayer to light irradiation quantity on the recording medium when saidrecording layer is in said amorphous state, said first ratio beinggreater than or equal to said second ratio, said first ratio and saidsecond ratio being calculated by a matrix method employing opticalconstants of constitutional materials of said respective layers.
 2. Themethod according to claim 1, wherein said recording layer has athickness less than 40 nm.
 3. The method according to claim 1, whereinsaid recording layer consists of a Ge--Sb--Te alloy.
 4. A method forrecording/reproducing optical information comprising:(a) laminatingsuccessively, on a substrate, a first dielectric layer, a phase changetype recording layer capable of changing reversibly between an amorphousstate and a crystalline state formed reversibly in response to laserbeam irradiation conditions, a second dielectric layer, a lightreflecting layer and a protection layer; and (b) selecting respectivethicknesses of said first dielectric layer, said recording layer, saidsecond dielectric layer, said light reflecting layer and said protectionlayer, in accordance with calculations, to provide a first calculatedratio of light absorption quantity in the recording layer to lightirradiation quantity on the recording medium when said recording layeris in said crystalline state and a second calculated ratio of lightabsorption quantity in the recording layer to light irradiation quantityon the recording medium when said recording layer is in said amorphousstate, said first ratio being grater than or equal to said second ratio,said first ratio and said second ratio being calculated by a matrixmethod employing optical constants of constitutional materials of saidrespective layers; (c) irradiating a single laser beam onto said opticalinformation recording medium in a power-modulated mode responsive toinformation signals; and (d) detecting the recorded and unrecorded stateof portions of said optical recording information medium by detectingreflectivity differences between recorded and unrecorded portions ofsaid optical recording medium.
 5. The method according to claim 4,wherein said recording layer has a thickness less than 40 nm.
 6. Themethod according to claim 4, wherein said recording layer consists of aGe--Sb--Te alloy.